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Old 18 May 2010, 16:48   #1
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Propeller choice help!!

I have a 50hp Johnson Four Stroke and the recommended propeller for this engine is a 11-3/8 x 14. On the engine at the moment i have a 11-3/8 x 13 propeller because thats what it came with when i brought it. However im looking at buying a new propeller because ive been told that it is under pitched and causing the engine to reach to higher RPM's, at the moment it gets upto 6150 RPM!

https://www.secure-clienthost.co.uk/...1&cat=0&page=1
I found this propeller because i was looking at going to a 15 inch pitch to increase the top speed, the only thing is that i cant find the right diameter propeller with a 15 inch pitch. Will the propeller above be ok for this engine or will the diameter size drop make a big difference seeing as it is over 1" smaller!!!

Thanks everyone for your help!!
alex
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Old 19 May 2010, 04:00   #2
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You need to find the max revs for your engine before you do anything.

Outboards will operate up to that without any problem and every hull and engine combination has potentially a different perfect prop, so having one recommended prop for one engine and every boat it could fit is a nonsense.
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Old 19 May 2010, 09:49   #3
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If your new prop is 1" diameter smaller, then it;s going to up the max attainable revs. Whether 15" pitch is enough to load it back up only an experiment will tell.

I had a "small" prop on mine when I got it, down I suspect to the boat the engine was on before it met my rib. First trip out with it and I chickened out at 6200 while still acceleratiing(max was a theoretical 5500) , and found a secondhand prop upped in diameter but same pitch. Maxed out at 5500 with 3/4" bigger diameter. I suispect 1" down be the last thing you want.


If it's pure straight line speed you're after, smaller diameter / big pitch is theoretically the way to go, BUT you will loose a lot of grip in corners, waves and hole shot.
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Old 19 May 2010, 12:30   #4
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thanks for the reply's!!!

The maximum rated RPM is 6500 but obviosuly i dont want to run it near that!!

Thanks for the detailed info there.... i guess i dont just want straight line speed as its a bit pointless! So i would want to keep the diameter or maybe go a slight bit larger because i think there is plenty of clearance at the moment, but will going up slightly in diameter improve or reduce the performance???

Thanks for your help, if i could find a decently priced right diameter and pitched one i would buy that but i cant find one
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Old 19 May 2010, 12:46   #5
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thanks for the reply's!!!

The maximum rated RPM is 6500 but obviosuly i dont want to run it near that!!
It that the max attainable or the recommended WOT rpm?

There should be 2 figures-eg 5500rpm to 6500rpm quoted for RPM.

You may find 6150 is ok yet, though it sounds high.
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Old 19 May 2010, 13:11   #6
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i believe its the max obtainable as it is 5500-6500 as you say. Just thinking i've found a 12.25 x 15 propeller, will that if it fits with the clearance be any good?? has there got to be a certain amount of room left between the plate and the prop tip??

thanks
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Old 19 May 2010, 13:51   #7
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i believe its the max obtainable as it is 5500-6500 as you say.
I'd stick with what you've got then.

However, if you really want to change prop go up an inch or 2 in pitch and use the standard OEM prop for the engine. Anything else won't give comparable results.
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Old 24 May 2010, 14:10   #8
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Nos is right, stay with the prop you have, as long as it is in good condition. The Johnson 50hp four stroke engine is a rebadged Suzuki. The recommended operating range is 5900 to 6500rpm. So at 6150 you are in the range without revving the nuts off it. If you tried the 12.25 x 15 prop you could loose at least 700rpm. If the prop even fitted with that diameter, this will drop you out of the rev range. The pick up will be worse also.
Aquatec.
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